Aspectos biológicos de interés para el potencial cultivo del pejerrey patagónico, Odontesthes hatcheri

Abstract

En esta tesis se abordó el estudio de tres aspectos principales de la biología del pejerrey patagónico, Odontesthes hatcheri, en relación a su potencial como recurso pesquero y su cultivo. En primer lugar, se analizó la identidad a nivel específico de pejerreyes provenientes de la región Andina, que corresponde al área de distribución original de O.hatcheri, y donde ha sido sembrado pejerrey bonaerense, Odontesthes bonariensis. En base al análisis de caracteres taxonómicos de clave, a la morfometría geométrica y a la genética mitocon-drial, se identificaron 436 individuos, capturados en 18 ambien-tes continentales y se describieron las combinaciones de los taxa encontrados. El análisis sobre secuencias de citocromo b permitió distinguir los haplotipos de las dos especies con un alto grado de divergencia genética (7%) entre ellas y la variación intraespecífica de este marcador fue muy baja en O. hatcheri. En sitios que correspondieron a la distribución original de O. hatcheri (Dyer,2000) se identificaron ejemplares de O. bonariensis y se hallaron individuos con incongruencias entre los caracteres morfológicos y los haplotipos mitocon-driales. En base a la información de genética mitocondrial, se detectaron en los Embalses Florentino Ameghino y Nihüil, y en la laguna Urre Lauquen, individuos que morfológicamente fueron identificados como O. hatcheri pero que portaban genoma mitocondrial de O. bonariensis, revelando posibles introgresiones génicas. En el lago Pellegrini se encontraron ejemplares con caracteres morfológicos híbridos. En un nivel intraespecífico, el análisis de la morfometría geométrica mostró diferencias en cuanto a la forma corporal según los ambientes. En relación al amplio rango latitudinal de distri-bución del pejerrey patagónico y a la heterogeneidad de hábitats, se buscaron factores ambientales de variación morfológica y se probó la relación entre la forma del cuerpo y parámetros físicos, químicos y geográficos de los sitios de captura. Entre ellos, la concentración total de fósforo y la altitud mostraron relaciones significativas, lo que podría estar asociado a las relaciones tróficas de los ambientes analiza-dos. En el estudio del crecimiento, se compararon parámetros de la curva de von Bertalanffy entre poblaciones de pe-jerreyes pertenecientes a seis embalses ubicados a lo largo de las cuencas de los ríos Neuquén y Limay. A partir de datos de frecuencia de talla se estimaron los parámetros mediante los métodos de Shepherd o SLCA (Shepherd s Length Composition Analysis), de ELEFAN (Electronic Length-Frequency Analysis) y del método de Po6 well-Whetherall en combinación con SLCA (P-W+SLCA). Los parámetros estimados fueron com-parados entre los ambientes. Los rangos de valores estimados para L fueron de 393 a 431 mm, para K de 0,22 a 0,53 y para t0 de -0,91 a -0,35. El índice de performance del crecimiento o índice , mostró valores de entre 2,579 y 2,993. Para ampliar el análisis, se compararon los parámetros de crecimiento con los obtenidos por otros autores en O. bonariensis, O. hatcheri y en la trucha arco iris (Oncorhynchus mykiss) y ésta última resultó ser la especie que crece en forma más veloz, con tallas asintóticas e índices de performance mayores. Los parámetros estimados para las poblaciones de los embalses estudiados no mostraron diferencias significativas con los parámetros encontrados en la bibliografía para O. hatcheri y O. bonariensis. Sin embargo, se pudo observar que estos valores fueron intermedios entre los estimados para ambas especies, lo que posiblemente estaría relacionado a las intro-gresiones de O bonariensis observadas en estos sitios.Las estructuras de talla de los pejerreyes mostraron ser similares entre los ambientes analisados, pero la distribución de fre-cuencias de tamaño del embalse Alicura presentó una primera cohorte de mayor tamaño que los demás ambientes, posible-mente debido a efectos de la predación de salmónidos sobre las tallas menores de pejerrey. El factor de condición mostró variación de acuerdo al ambiente, al sexo y al estadio repro-ductivo, siendo mayor en el estadio de desove. Los períodos de desove correspondieron mayormente a los meses de prima-vera y para el embalse El Chañar primavera y otoño. La talla promedio de madurez sexual de las hembras en estos seis ambientes fue de 173 a 206 mm LT, que correspondió a las edades menores a un año de los embalses Chañar y Arroyito, respectivamente. La tercera parte de esta tesis se enfocó en el estudio de los efectos del estrés en el pejerrey patagónico bajo distintas prácticas ligadas al cultivo, mediante medicio-nes de cortisol plasmático y expresión hepática del ARNm para IGF-1. Se exploró frente a diferentes salinidades, la relación del cortisol plasmático con parámetros fisiológicos, como la osmolaridad plasmática, el hematocrito, y el contenido relativo de agua corporal. En una primera experiencia se sometieron pejerreyes a captura, transporte y cautiverio. Se observó una regresión cuadrática entre los niveles plasmáticos de cortisol y el tiempo de exposición al tratamiento ligado a una práctica de cultivo. Los niveles máximos de cortisol plasmático y también de IGF-1 ARNm hepático se observaron en cautiverio, seis días después de la captura. En la segunda experiencia, la concentración de sal de 0,5 %, disminuyó el estrés en condi-ciones de cautiverio, con una mortalidad nula. Por el contra-rio, la concentración de 2 % fue letal y la exposición gradual a esta concentración generó una mortalidad del 50%.Se observó que los efectos de los factores de estrés tales como la captura, el transporte, y el confinamiento pueden ser evaluados en O.hatcheri mediante los niveles plasmáticos de cortisol y que el estrés agudo que generan puede estar relacionado con un aumento en la expresión relativa de ARNm para el IGF-1. En este sentido, la selección de la salinidad cercana a concentraciones de 0,5 % mitiga los efectos del estrés y mejora los niveles de supervivencia de O.hatcheri en cautiverio.

This thesis considers three aspects of the biology of the Patagonian silverside, Odontesthes hatcheri, regarding its potential for fishery and culture. Firstly, we analyzed the specific identity of silversides from the Andean region, the original range of O. hatcheri in which the Bonaerensean pejerrey Odontesthes bonariensis was stocked. Based on the analysis of key taxonomic characters, geometric morphometrics and mitochondrial genes, we identified 436 individuals captured at 18 sites and were able to describe taxa combinations. The analysis of cytochrome b sequences showed a high degree of genetic divergence (7%) between both species. Intraspecific variation was very low in O. hatcheri. We identified sites with O. bonariensis and some individuals with inconsistencies between morphology and mitochondrial haplotypes, which suggest genetic introgression. In Lake Pellegrini, specimens with hybrid morphology were found. At intraspecific level, the geometric morphometrics showed different body shapes among sites. Regarding the latitudinal range of distribution of the Patagonian silverside and the habitat heterogeneity, we looked for environmental sources of morphological variation and tested the relationship between body shape and physical, chemical and geographic parameters. The total phosphorus concentration and the altitude showed significant relationships that would be associated with trophic relationships in the analyzed environments. Secondly, we compared growth parameters of von Bertalanffy curve among silverside populations from six reservoirs located along Limay and Neuquén Rivers. The parameters were estimated from length-frequency data using Shepherd or SLCA method (Shepherd s Length Composition Analysis), ELEFAN method (Electronic Length-Frequency Analysis), and the Powell-Whetherall method in combination with SLCA (P -W + SLCA). Ranges of estimated values were 398 to 432 mm for L, 0.37 to 0.535 for K and -0.8 to -0.612 for t0. The growth performance index ( ) ranged from 2.614 to 3.107. To expand the analysis, these values were compared with those obtained by other authors in O. bonariensis, O. hatcheri and rainbow trout (Oncorhynchus mykiss). In our study, the estimated parameters were within ranges found in the literature for the Patagonian silverside. Size structures of the silversides were shown to be similar between reservoirs, but in Alicura reservoir, effects of salmonid predation on smaller sizes could be observed. The condition changed according to reservoir, sex, and gonadal status, being greater at breeding. Spawning periods corresponded mainly to spring and, in the El Chañar reservoir, to spring and autumn. The mean size at sexual maturity of females in the six reservoirs ranged from 173 to 206 mm TL, corresponding to ages less than 1 year at El Chañar and Arroyito reservoirs. Thirdly we studied the effects of stress on the Patagonian silverside under different practices as regards its possible cultivation. We explored different salinities to asses the levels of plasma cortisol and physiological parameters related to osmoregulation, such as plasma osmolality, hematocrit, and relative body water content. The relationship between stress and growth was studied by measuring the hepatic expression of IGF-1 mRNA. In a, first experiment silversides were subjected to capture, transport and captivity. Quadratic regression was observed between plasma levels of cortisol and time. Higher levels of plasma cortisol, as well as hepatic IGF-1 mRNA were observed six days after capture. In a second experiment, a salt concentration of 0.5%, decreased stress in captivity, with no consequent mortality. By contrast, a 2% concentration was lethal and the gradual exposure to this concentration resulted in a 50% mortality.We observed that the effects of stressors such as capture, transport, crowding, and confinement can be evaluated in O. hatcheri by plasma levels of cortisol. Acute stress can be related to an increase in the relative expression of mRNA for IGF-1. Finally, careful selection of salinity near 0.5% mitigates the effects of stress and improves survival rates of O.hatcheri in captivity

This thesis considers three aspects of the biology of the Patagonian silverside, Odontesthes hatcheri, regarding its potential for fishery and culture. Firstly, we analyzed the specific identity of silversides from the Andean region, the original range of O. hatcheri in which the Bonaerensean pejerrey Odontesthes bonariensis was stocked. Based on the analysis of key taxonomic characters, geo-metric morphometrics and mitochondrial genes, we identi-fied 436 individuals captured at 18 sites and were able to describe taxa combinations. The analysis of cytochrome b sequences showed a high degree of genetic divergence (7%) between both species. Intraspecific variation was very low in O. hatcheri. We identified sites with O. bonariensis and some individuals with inconsistencies between morpho-logy and mitochondrial haplotypes, which suggest genetic introgression. In Lake Pellegrini, specimens with hybrid morphology were found. At intraspecific level, the geome-tric morphometrics showed different body shapes among sites. Regarding the latitudinal range of distribution of the Patagonian silverside and the habitat heterogeneity, we looked for environmental sources of morphological variation and tested the relationship between body shape and physical, chemical and geographic parameters. The total phosphorus concentration and the altitude showed significant relationships that would be associated with trophic relationships in the analyzed environments. Secon-dly, we compared growth parameters of von Bertalanffy curve among silverside populations from six reservoirs located along Limay and Neuquén Rivers. The parameters were estimated from length-frequency data using Shepherd or SLCA method (Shepherd s Length Composition Analysis), ELEFAN method (Electronic Length-Frequency Analysis), and the Powell-Whetherall method in combination with SLCA (P -W + SLCA). Ranges of estimated values were 398 to 432 mm for L, 0.37 to 0.535 for K and -0.8 to -0.612 for t0. The growth performance index ( ) ranged from 2.614 to 3.107. To expand the analysis, these values were compared with those obtained by other authors in O. bonariensis, O. hatcheri and rainbow trout (Oncorhynchus mykiss). In our study, the estimated parameters were within ranges found in the literature for the Patagonian silverside. Size structures of the silversides were shown to be similar between reservoirs, but in Alicura reservoir, effects of salmonid predation on smaller sizes could be observed. The condition changed according to reservoir, sex, and gonadal status, being greater at breeding. Spawning periods corresponded mainly to spring and, in the El Chañar reservoir, to spring and autumn. The mean size at sexual maturity of females in the six reservoirs ranged from 173 to 206 mm TL, corresponding to ages less than 1 year at El Chañar and Arroyito reservoirs. Thirdly we studied the effects of stress on the Patagonian silverside under different practices as regards its possible cultivation. We explored different salinities to asses the levels of plasma cortisol and physiological parameters related to osmoregulation, such as plasma osmolality, hematocrit, and relative body water content. The relationship between stress and growth was studied by measuring the hepatic expression of IGF-1 mRNA. In a, first experiment silver-sides were subjected to capture, transport and captivity. Quadratic regression was observed between plasma levels of cortisol and time. Higher levels of plasma cortisol, as well as hepatic IGF-1 mRNA were observed six days after capture. In a second experiment, a salt concentration of 0.5%, decreased stress in captivity, with no consequent mortality. By contrast, a 2% concentration was lethal and the gradual exposure to this concentration resulted in a 50% mortality.We observed that the effects of stressors such as capture, transport, crowding, and confinement can be evaluated in O. hatcheri by plasma levels of cortisol. Acute stress can be related to an increase in the relative expression of mRNA for IGF-1. Finally, careful selection of salinity near 0.5% mitigates the effects of stress and improves survival rates of O .hatcheri in captivity